Ion Source and Injector Experiments at the HIF/VNL

1. Ion Source and Injector Experiments at the HIF/VNL J. W. Kwan, D. Baca, E. Henestroza, J. Kapica, F. M. Bieniosek, W.L. Waldron, J.-L. Vay, S. Yu, LBNLG.A. Westenskow, D. P. Grote, E. Halaxa, LLNLI. Haber, Univ. of MarylandL. Grisham, PPPL HIF Symposium, Princeton, NJJune 7, 2004

2. This talk is dedicated to the amazing Cicada In the hope that the HIF symposium 2021, Princeton, NJ will tell the story of heavy ion beams achieving fusion

3. A summary of main experiments

4. Experiments on STS-500 to study beam optics 500 kV, 17 ms pulse, 1.0 ms rise time

5. Experimental Apparatus Slit scanners: 2 mils, 17.8 cm apart 10-cm diameter K+ Al-Si source with Pierce electrode For Beam Imaging, use: • Kapton • 100 mm Alumina scintillator Faraday cup with electron suppressor using a honeycomb bottom

6. Good agreement between experimental results and simulation predictions 10 cm source, 21 cm diode gap,Space charge limited mode Experimental results Warp simulations 150 kV48A heater Emittance taken here

7. The emission-limited under-dense beam did not show much aberration

8. Aperturing the large beam 7.5 cm aperture aperture 5.0 cm aperture Brightness comparison

9. The apertured beam showed no aberrations 7.5 cm aperture Optical image from the alumina scintillator taken with a gated camera Integrated current density profile (compares to a slit cup measurement)

10. Time-dependent adaptive-mesh simulation shows how to achieve a fast rise time Current at Faraday cup Applied Diode Voltage Red--expt. data;black--simulation • The current pulse rises faster than the applied voltage pulse. • Capacitive coupling softens the signal rise time. • One dimensional theoretical model: • Example: 50ns/350ns

11. x-z y-z Merging high density beamlets is an innovative approach to build compact multi-beam HIF injector • Achieve high current, and high average current density • Minimize the injector and matching section size for a compact multi-beam HIF driver system

12. WARP-3D simulation to study emittance growth Configuration Phase 91 beamlets (each semi-Gaussian, 0.006 A, 0.003 π-mm-mrad, 160k particles), 1.2-1.6 MeV, 1024x1024, 1 cm/step After the beamlets are merged, the emittances settle down at about 1.0 pi-mm-mrad. Emittance is optimized if the number of beamlets is large and the beamlets are slight converging, but only weakly dependent on the emittance of each beamlet. 1.9 m 0.5 m past column 4.1 m 39.9 m 1.9 m 4.1 m

13. Testing Plasma Source on STS-100 RF-driven 26 cm diam. multi-cusp source inside ceramic insulator 500ms, 20kW, ~ 10 MHz Compact RF oscillator

14. Characterization of the RF plasma source 18 kW of 13 MHz RF,multicusp Argon plasma source at optimum pressure of 2 mTorr Multi-aperture extracts61 beamlets at 100 mA/cm2 using high gradient insulator Einzel lens to focus beamlets and examine charge exchange loss

15. RF plasma source beamlets results Achieve 100 mA/cm2 90% Ar+ < 0.5% low energy component Electrostatic energy analyser

16. Full Gradient test on STS-500 will begin this month This experiment will confirm full current density, its uniformity, and voltage gradient across vacuum gap.

17. Merging Beamlets test will begin in September • Apparatus is full scale in dimension, but1/4 scale in voltage,so 1/8 in current. • The experiment will study emittance growth physics, beam matching parameters, and beam halos. • Success in this experiment will establish the basis for building a (future) driver-scale injector.

18. Negative ion beams is an innovative idea in response to the gas and electrons problem • Avoid the problem of electrons being trapped in positive ion beams • No charge exchange problem to cause energy dispersion • Low ion temperature for both negative and positive halogen ions • Can be efficiently converted to atomic neutrals by laser photo-detachment, if this can be of advantage to the final focusing at the fusion chamber.

19. Negative ion sources for HIF Drivers • We have already demonstrated 45 mA/cm2 of pure Cl- ions with relatively low co-extracted electrons (7:1) from a single aperture. • Current density scaled almost linearly with RF power (12.56 MHz). • Current density of Cl+ ~ 1.3 x Cl-. • A new experiment will run on STS-100 this summer to examine the negative ion production from a large source, measure emittance, and form an array of beamlets.

20. Solenoid Ion Source 30kV -350kV 0V The accel-decel injector is an innovation to meet our HEDP challenge: build a low energy high current driver to hit target • In an accel-decel injector, a long pulse is compressed when decelerates into a solenoid, the Super-High l (line charge density) bunch is then accelerated without expansion. The situation is similar to loading passengers into a roller coaster train. l = I/v 10A x 100ns= 0.3m x 3.3 mC/m • At 3.3 mC/m, the HEDP l is > 10x the present HCX experiment. • Longitudinal emittance can coupling to transverse emittance • Possible compression limit when the bunch’s forward kinetic energy becomes comparable to the beam potential.

21. A proof-of-principle Super-High l experiment 60 cm solenoid located 5 cm from ground plate (winding:7.7cm ID, 9.2 cm OD,1 Mega Amp-Turn) NDCX-1 Bz/100(Tesla) 30 kV -220 kV -35 kV -55 kV 0 kV K+ Gun (using Al-Si source) E.H.20.MAY.04

22. Conclusion • Several ion source/injector experiments at the HIF/VNL are aimed at:-- supporting on-going HIF needs, -- developing future HIF driver, -- innovative concepts (high J, high l, fast rise, negative ions) • In response to funding difficulty, the injector test facility at LLNL is scheduled to terminate in March 2004. • We hope STS-100 can be moved to LBNL to continue ion source development.

23. After Thought What is unchanged is the constantly changing direction.What is certain is the permanently uncertain state.